With the development of Internet, e-mail has been widely used. As e-mail service is used as a new means of communication instead of an existing postal service, some Internet portal sites provide free mail accounts. Almost of all e-mail users are provided with at least one e-mail account (or e-mail address) by such Internet portal sites and use e-mail for various purposes.
Since many e-mail users have at least one e-mail address, a sender needs to know at least one e-mail address of a recipient among a plurality of e-mail addresses possessed by the recipient and needs to know a new address when the recipient's address is changed. In other words, all of e-mail users are bothered by needing to know an e-mail address of a recipient in advance to send a message to the recipient.
Accordingly, new service allowing an e-mail user (i.e., an e-mail sender) to send message to a recipient by e-mail even if the e-mail sender does not know the recipient's e-mail address is desired.